Deputy White House National Security Adviser Ben Rhodes said on CNN that no classified information had been compromised, while declining to link any attacks to Russia.

“We do not believe our classified systems were compromised,” Rhodes said Tuesday. He said the White House is “constantly updating” its unclassified system and that personnel are told to act as if information on that network could be compromised by hackers.

U.S. intelligence officials say the pace and sophistication of Russian-sponsored attacks have increased as tension over Ukraine has grown and the U.S. has imposed economic sanctions on Russia.

The White House said in October that it had identified potentially threatening activity on its computer network and had taken immediate steps to address it. At the time, two U.S. officials who sought anonymity said cybersecurity specialists suspected that Russian government or criminal hackers were responsible for the intrusion.

Mark Stroh, a White House National Security Council spokesman, said in an e-mail Tuesday that CNN’s report didn’t refer to any new incident.

Very Seriously’

“Any such activity is something we take very seriously,” Stroh said. “In this case, as we made clear at the time, we took immediate measures to evaluate and mitigate the activity.”

The U.S. said last year it suspected the Russian government or criminal hackers of being behind the October attack on a White House computer system.

Some U.S. officials investigating the intrusion suspect that the hackers may have gotten into unclassified White House systems after gaining entry at the State Department, where officials regularly use e-mail to communicate with colleagues, according to U.S. officials who spoke on condition of anonymity.

The State Department incident late last year proved difficult to root out, the officials said, because the hackers kept revising their tools to foil defensive efforts.